Heat and mass transfer in vertical tubular bubble absorbers for ammonia-water absorption refrigeration systems

A model is developed for calculation of simultaneous heat and mass transfer processes in vertical bubble absorbers as used for ammonia-water absorption refrigeration systems. Some preliminary experiments have been performed in an absorber without heat removal. The results from these experiments are compared with the literature and give a first indication about the methods for prediction of the absorption process. Experiments have also been performed with simultaneous heat removal. The internal diameters of the absorbers tested were 10.0, 15.3, and 20.5 mm. The mass transfer coefficients resulting from these experiments are correlated by a modified Sherwood relation. An interative procedure is presented which allows design of vertical tubular bubble absorbers for ammonia-water absorption refrigeration systems.     

A study on numerical simulations and experiments for mass transfer in bubble mode absorber of ammonia and water

An absorber is a major component in the absorption refrigeration systems, and its performance greatly affects the overall system performance. In this study, both the numerical and experimental analyses in the absorption process of a bubble mode absorber were performed. Gas was injected into the bottom of the absorber at a constant solution flow rate. The region of gas absorption was estimated by both numerical and experimental analyses. A higher gas flow rate increases the region of gas absorption. As the temperature and concentration of the input solution decrease, the region of gas absorption decreases. In addition, the absorption performance of the countercurrent flow was superior to that of cocurrent. Mathematical modeling equations were derived from the material balance for the gas and liquid phases based on neglecting the heat and mass transfer of water from liquid to gas phase. A comparison of the model simulation and experimental results shows similar values. This means that this numerical model can be applied for design of a bubble mode absorber.     

The effect of oil-droplet on bubble absorption performance in binary nanoemulsions

The objectives of this paper are to study the absorption characteristics of NH₃ bubbles in binary nanoemulsions and to quantify the effect of oil-droplet on the bubble absorption performance. C₁₂E₄ and Tween20 are used as the surfactants and n-decane oil is added into NH₃/H₂O solution to make the binary nanoemulsion. The initial concentration of NH₃/H₂O solution and the concentration of oil are considered as the key parameters. The absorption rates are calculated by measuring the inlet and outlet mass flow rates per a given time period. In addition, the droplet size in the binary nanoemulsion is measured by the particle size analyzer (ELS-Z, OTSUKA ELECTRONICS). It is found that the effective absorption ratio for 2.0 vol% oil and 14.3 wt% NH₃/H₂O becomes 17% higher than that for the base fluid.     

Effect of vapor flow on the falling-film heat and mass transfer of the ammonia/water absorber

Falling-film heat and mass transfer in an absorber can be influenced by the motion of the surrounding refrigerant vapor. In this study, the effect of the vapor flow direction on the absorption heat and mass transfer has been investigated for a falling-film helical coil absorber which is frequently used in the ammonia/water absorption refrigerators. The heat and mass transfer performance was measured for both parallel and countercurrent flow. The experiments were carried out for three different solution concentrations (3, 14, and 30%). The vapor in equilibrium with the solution is supplied to the test section. It is found that the falling-film heat and mass transfer is deteriorated in the countercurrent flow if the specific volume of the vapor solution is large. For the countercurrent flow, the high velocity of the vapor due to large specific volume seems to cause the unfavorable distribution of falling-film and reduce the heat and mass transfer performance of the ammonia absorber. The effect of vapor flow direction decreased with increasing concentration of ammonia solution since the specific volume of the ammonia vapor which is in equilibrium with the solution becomes smaller and the vapor velocity becomes lower.     

Heat and mass transfer characteristics in a spray chamber

This paper presents a one-dimensional mathematical model for heat and mass transfer of water droplets in a spray chamber. The model includes drop size distribution and velocity of the droplets generated by a nozzle of inlet diameter 3.2 mm. By using the conservation of mass and energy, the changes in water temperature, air temperature and humidity along the spray cone in the spray chamber can be calculated. This model is tested with two different water mass flows. The results look reasonable from practical point of view and they also show that higher water mass flow results in a higher air temperature drop and higher humidity.     

Numerical design of ammonia bubble absorber applying binary nanofluids and surfactants

The objectives of this paper are to analyze the combined heat and mass transfer characteristics for the ammonia bubble absorption process and to study the effects of binary nanofluids and surfactants on the absorber size. The ammonia bubble absorbers applying binary nanofluids and surfactants are designed and parametric analyses are performed. In order to express the effects of binary nanofluids and/or surfactants on the absorption performance, the effective absorption ratios for each case are applied in the numerical model. The values of the effective absorption ratio are decided from the previous experimental correlations. The kinds and the concentrations of nano-particles and surfactants are considered as the key parameters. The considered surfactants are 2-ethyl-1-hexanol (2E1H), n-octanol, and 2-octanol and nano-particles are copper (Cu), copper oxide (CuO), and alumina (Al₂O₃). The results show that the application of binary nanofluids and surfactants can reduce the size of absorber significantly. In order to reach 16.5% ammonia solution under the considered conditions, for example, the addition of surfactants (2E1H, 700 ppm) can reduce the size of absorber up to 63.0%, while the application of binary nanofluids (Cu, 1000 ppm) can reduce it up to 54.4%. In addition, it is found that the effect of mass transfer resistance is more dominant than that of heat transfer resistance. That is, the enhancement of mass transfer performance is more effective than that of heat transfer performance.     

An experimental and theoretical study of the influence of surfactant on the preparation and stability of ammonia-water nanofluids

Two types of nanofluids are obtained by adding the mixture of carbon black nanoparticles with emulsifier OP-10, and Al₂O₃ nanoparticles with sodium dodecyl benzene sulfonate (SDBS) in the ammonia-water solution, respectively. The dispersion stability of the prepared nanofluids in different mass fractions of surfactants is investigated by the light absorbency ratio index methods. The results show that with the increase of mass fraction of surfactant, the stability of carbon black nanofluid is improved firstly and then is exacerbated, while the stability of Al₂O₃ nanofluid is exacerbated firstly, then is improved, and then is exacerbated again. The influences of surfactant on the stability of ammonia-water nanofluids abide by the monolayer adsorption theory or electric double layer adsorption theory. Finally, the theoretical surfactant mass fractions required in the preparation of ammonia-water nanofluids are calculated by simplifying the dispersion models and the results are in accordance with experimental results.     

Development of a slug flow absorber working with ammonia-water mixture: part I—flow characterization and experimental investigation

This study deals with an experimental investigation for a counter-current slug flow absorber, working with ammonia–water mixture, for significantly low solution flow rate conditions that are required for operating as the GAX (generator absorber heat exchanger) cycle. It is confirmed that the slug flow absorber operates well at the low solution flow rate conditions. From visualization results of the flow pattern, frost flow just after the gas inlet, followed by slug flow with well-shaped Taylor bubble, is observed, while dry patch on the tube wall are not observed. The liquid film at the slug flow region has smooth gas–liquid interface structure without apparent wavy motion. The local heat transfer rate is measured by varying main parameters, namely, ammonia gas flow rate, solution flow rate, ammonia concentration of inlet solution and coolant inlet conditions. The heat transfer rate while absorption is taking place is higher than that after absorption has ended. The absorption length is greatly influenced by varying main parameters, due to flow conditions and thermal conditions.     

Ammonia-water desorption heat and mass transfer in microchannel devices

This paper presents the results of an experimentally validated model for the prediction of local heat and mass transfer rates in a microchannel ammonia-water desorber. The desorber is an extremely compact 178 mm × 178 mm × 0.508 m tall component capable of transferring the required heat load (∼17.5 kW) for a residential heat pump system. The model predicts temperature, concentration and mass flow rate profiles through the desorber, as well as the effective wetted area of the heat transfer surface. Previous experimental and analytical research by the authors demonstrated the performance of this same microchannel geometry as an absorber. Together, these studies show that this compact geometry is suitable for all components in an absorption heat pump, which would enable the increased use of absorption technology in the small-capacity heat pump market.     

Effect of graphene oxide coating on bubble dynamics and nucleate pool boiling heat transfer

Materials coating has been proved to be an effective mean to increase the number of active nucleating sites, and therefore generate more vapor bubbles and lead to better pool boiling heat transfer performance. In this work, graphene oxide (GO) is coated on a boiling surface by self-assembly method, to enhance critical heat flux (CHF). The pool boiling is carried out on a smooth copper surface to study the effect of GO coating using distilled water as the working fluid along with bubble dynamic visualization. GO coating facilitates bubble nucleation by providing numerous microscale cavities. The visualization investigation of bubble dynamic behavior shows that the CO-coated surface exhibits a higher bubble departure frequency, a smaller bubble departure diameter and smaller bubble diameters in the pool, indicating greatly enhanced heat transfer effects. Meanwhile, the GO-coated surface exhibited a smaller contact angle than the copper surface, revealing that surface becomes more hydrophilic after GO coating. Consequently, GO-coated surface with a coating time of 4 h provides a CHF of 224.3 W/cm² and a heat transfer coefficient (HTC) of 8.79 W/(cm²·K), representing an improvement of 94.0% in CHF and 83.5% in HTC compared to smooth copper surface.     

Heat and mass transfer between a boiling liquid and bubbles

A method is proposed for calculating the flow of a boiling liquid with bubbles through vertical pipes (containers).Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 23, No. 5, pp. 859–867, November, 1972.     

Heat and mass transfer in a submerged axisymmetric twisted jet

The excess-temperature distribution in a twisted flow is obtained for numbers Pr≠ 1, and the distribution of the concentration of a gaseous impurity is investigated.     

The effect of micro-scale surface treatment on heat and mass transfer performance for a falling film H2O/LiBr absorber

The objectives of this paper are to develop a new method of wettability measurement, to study the effect of micro-scale surface treatment on the wettability across horizontal tubes and to apply it for numerical analysis of heat and mass transfer in a H₂O/LiBr falling film absorber. Three types of tubes with roughness are tested in a test rig. Inlet solution temperature (30–50 °C), concentration (55–62 wt.% of LiBr) and mass flow rate (0.74–2.71 kg/min) are considered as key parameters. Reynolds number ranged from 30 to 120 by controlling the inlet mass flow rate. The wettability on the roughened tubes was higher than that for the smooth tubes. The wettability decreased linearly along the vertical location but was proportional to the solution temperature and mass flow rate. The experimental correlations of the wettability for the smooth and the roughened tubes were developed with error bands of ±20 and ±10%, respectively. These are used for the heat and mass transfer analysis of absorbers with micro-scale hatched tubes.     

Simulation of bubble motion in a compressible liquid based on the three dimensional wave equation

A simulation model for bubble motion in a compressible liquid is developed based on the linear wave equation. At the initial stage, the bubble is assumed to be spherical and the wave equation is simplified as a one dimensional ordinary equation in the radial direction through the prophase approximation. When time becomes much larger than that required for a disturbance to travel across the bubble at the speed of the sound, the obtained integral equation is approximated by keeping the term of the first order in terms of the characteristic Mach number, through the anaphase approximation. An equation is introduced to unify the approximations in these two phases, which is then used over the entire simulation period. The problem at each time step is solved by a three dimensional boundary element method. The convergence study has been first taken with meshes and time steps. Comparison is made with the analytical solution for spherical bubble in compressible liquid and good agreement is found. Further comparison is made for a bubble in an incoming acoustic wave. Extensive simulations are then made for a bubble in various conditions, including the cases with solid boundary effect, free surface effect, buoyancy effect, as well as for interactions between bubbles.     

Heat and mass transfer during sublimation and condensation in a conical annulus

A solution is given for thermostatic control of conical and spherical objects under conditions of one-sided radiative heating in a vacuum.     

Heat and mass transfer at a vitroplastic surface in a high-temperature air stream

Analyzed are the basic laws which govern the decomposition of vitrographitic materials and the effect of superficial carbon burnout on the wear characteristics of such materials.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 3, pp. 407–413, March, 1973.     

Heat and mass transfer in bundles of finned rods

A generalized relation for calculating the effective turbulent diffusion coefficients in bundles of finned rods is derived on the basis of an analysis of the results of interchannel cross-mixing studies by various authors.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 3, pp. 357–363, September, 1984.     

Condensation heat transfer and pressure drop characteristics of CO2 in a microchannel

The condensation heat transfer coefficient and pressure drop of CO₂ in a multiport microchannel with a hydraulic diameter of 1.5 mm was investigated with variation of the mass flux from 400 to 1000 kgm⁻²s⁻¹ and of the condensation temperature from −5 to 5 °C. The heat transfer coefficient and pressure drop increased with the decrease of condensation temperature and the increase of mass flux. However, the rate of increase of the heat transfer coefficient was retarded by these changes. The gradient of the pressure drop with respect to vapor quality is significant with the increase of mass flux. The existing models for heat transfer coefficient overpredicted the experimental data, and the deviation increased at high vapor quality and at high heat transfer coefficient. The smallest mean deviation of ±51.8% was found by the Thome et al. model. For the pressure drop, the Mishima and Hibiki model showed mean deviation of 29.1%.